524 
fect filtration and to check slightly the leakage of small 
organisms through the silk. In Spoon River, on the other 
hand, the plankton is scant and the silt mainly of fine loam 
and clay in suspension, so that the silk net rarely clogs and 
the escape of small planktonts is but slightly impeded. This 
loss by leakage is far more significant in the case of Spoon 
River than it is in the Illinois, for the escaping plankton con- 
stitutes a relatively larger proportion of Spoon River’s total 
product than an equally large or even greater loss would form 
of the total product of the [linois. Again, the quantity of 
silt is both relatively and absolutely much greater in the waters 
of Spoon River than it is in the Ilinois, and it is of a different 
nature. This greatly increases the difficulty of maintaining 
any uniformity of standard in the estimation of the silt con- 
tent of the plankton catches in the two streams. These sources 
of error, although considerable, do not, however, invalidate the 
conclusions here drawn regarding the relative productivity of 
the two-streams. They are still patent within any reasonable 
limit of error. 
The plankton of Spoon River is very much less than that 
of the Illinois. The average amount present in a cubic meter 
of water in Spoon River, as shown by the average of the 
amounts in thirty-six collections made between August 18, 1896, 
and March 7, 1899, is .465 cubic centimeters. This average 
amount is reduced to 0.191 cubic centimeters if the two collec- 
tions of September 11 and 30, 1897, are omitted. This average is 
still more reduced if we omit the collections from the last of 
August, 1897, to the close of the year—a period of exceptional 
and prolonged low water. The omission of these four months 
lowers the average to 0.044 em.’ per m.? of water,an amount 
which more truly represents the normal contributions of the 
tributary to the main stream in years of rainfall normal both 
in quantity and distribution. 
The average plankton content of a cubic meter of Illinois 
River water, as shown by the average of 235 collections made 
between June 12, 1894, and March 28, 1899, is 2.19 cubic centi- 
